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Two years ago, the physicist Neil Gershenfeld argued that the next great digital revolution would come not in computing or communication but in fabrication. Today, 3-D printing, which uses digital data to rapidly construct physical objects, is more accessible than ever. Automakers are placing the latest printing devices on factory floors, design students are learning they can make virtually anything out of plastic, and scientists are experimenting with human tissue. Last year, Australian biologists at the University of Queensland created a functioning kidney from scratch, albeit in miniature; they are now working on printing such organs by layering tiny sheets of living cells.

Yet just as the world is beginning to grasp the implications of the 3-D revolution, researchers are proposing an upgrade. Their work suggests that the true promise of digital fabrication lies with a fourth dimension -- in printing objects programmed to change over time. Whereas 3-D objects are static and require human control, 4-D objects can react to their environment autonomously, assembling, repairing, or transforming themselves. Advances in this realm will rely less on printing technology than on materials themselves. At MIT, the architect Skylar Tibbits combined a 3-D printer with a novel combination of materials to create a wirelike object that when dropped in water changed its shape to form the letters “MIT.” It was the pairing of a water-absorbing polymer material with a basic plastic that made the effect possible.

The potential applications, of course, are endless. And they could prove especially significant on large scales. Consider underground pipe systems, long troubled by fixed capacities and costly maintenance. With programmable materials, individual pipes could adapt to shifting conditions, expanding or contracting to adjust capacity and flow rates. They might even repair themselves when damaged or disintegrate once out of use.

Or think of the possibilities for the defense industry. The U.S. Army has already started using 3-D printing, deploying a team to develop new gear on the frontlines in Afghanistan. And in an effort to